Display devices such as knobs for draught beer taps and taps for dispensing other liquids have been made in various sizes and shapes in an attempt to achieve point of purchase promotion of a particular product. While these proposals have met with some success, it is more desirable to provide some eye catching feature with the display device in order to attract customer attention. For example, U.S. Pat. No. 3,762,086, issued to Horbinski, depicts a rotatable tap knob. Proposals such as a illuminatable tap knobs have previously been considered inadequate to attract customer attention due to the need to supply a power source, complexity and cost.